A mobile unit roaming through a cellular communications system experiences a varying quality of communications as it moves nearer or further away from a serving base station. The process of `handoff`, where a mobile is handed off to an alternative base station that can better serve the mobile, can be divided into three stages: `trigger`, where it is decided that the quality of communications has changed enough to consider handoff; `selection`, where measurements of alternative base stations are made; and `decision` where a decision is made to handoff a mobile from the presently serving base station to the best available alternative base station.
In early cellular systems, all stages of handoff were performed by base stations of the system. In particular, serving and possible alternative base stations measured the quality of communications with a mobile. This clearly places a burden on the network, a burden which increases as the number of mobiles that must be monitored increases. The progression to digital time-division multiple access (TDMA) cellular systems has further increased the monitoring burden of the network. With an analogue cellular system such as AMPS a base station must tune to the frequency that the mobile is transmitting on and make a measurement of communications quality--typically received signal strength (RSS). With a digital system such as TDMA the base station must similarly tune to the frequency that the mobile is transmitting on, but additionally must find, and achieve synchronisation with, the particular time-slot that the mobile is using, and then make a measurement of the communications quality typically RSS or Bit Error Rate (BER).
Mobile-assisted handoff techniques reduce the amount of monitoring network that the network must perform by using measurements made by the mobile to assess potential alternative base stations.
According to one conventional mobile-assisted handoff technique, when a mobile begins communication with a base station on a TDMA voice channel the mobile is directed to begin signal strength measurements of the current forward voice channel as well as the control channels of surrounding cells. The mobile averages a number of the most recent measurements and reports this average RSSI measurement to the serving base station every second. When the averaged RSSI of the forward voice channel drops below a certain threshold and the mobile reports that at least one of the control channels of an adjacent cell is at an acceptable RSSI, a handoff trigger is generated. A target cell is considered from the best candidates reported by the mobile. Each of the best candidate cells are then requested to make a measurement of communications quality from the mobile using their locate receiver. The base station offering the best measurement that also has an available voice channel is selected, and a handoff attempt is made to that base station. This offers the advantage that only a small set of base stations, those identified by the mobile as offering the best signal, need measure the mobile.
However, if any of the candidate base stations or the serving base station does not have a locate receiver then the raw signal strength measurements made by the mobile are used directly to determine the target cell for handoff.
There are a number of disadvantages with the technique just described. Firstly, even the limited use of digital locate receivers to measure mobiles places a significant monitoring demand on the base stations and the supporting signalling network. Secondly, if locate receivers are unavailable, or if it is decided not to use them, then mobile signal measurements of adjacent control channels are used. These measurements do not account for a number of important issues, particularly the differences between control and voice channel powers and the power that a base station is delivering at the moment that a measurement is made.
Accordingly, there is a need to reduce the amount of monitoring that a network must carry out at handoff and a need to make mobile measurements a more reliable basis on which to handoff a mobile.
There is a further problem in conventional mobile-assisted handoff systems that both the base station and the mobile may initiate the `trigger` stage. The base station takes its own measurements of call quality and triggers handoffs in response to those measurements. In such circumstances, a handoff triggered by a base station on cell A may determine that cell B is the best for the mobile, while a handoff triggerd by a mobile in cell B may determine that cell A is the best. This can lead to a `ping-pong` effect between the mobile and base station This unnecessarily increases the processing burden on the system and degrades the overall call quality.
It is an object of the present invention to provide an improved handoff technique that reduces or overcomes these drawbacks.
It is a further object of the invention to provide an improved mobile cellular communications system.